North Korea fired two medium-range ballistic missiles Sunday, Seoul's military said, days after Pyongyang announced a successful test of a solid-fuel motor for a new weapons system.

Military tensions on the Korean peninsula have risen sharply this year as Pyongyang has carried out an unprecedented blitz of weapons tests, including the launch of its most advanced intercontinental ballistic missile ever last month.

South Korea's Joint Chiefs of Staff said it detected two medium-range ballistic missiles that had been fired from the Tongchang-ri area in North Pyongan province.

The missiles were fired from 11:13 am (0213 GMT) to 12:05 pm into the East Sea, it said, referring to the body of water also known as the Sea of Japan.

They were fired on a "lofted" trajectory and flew around 500 kilometres (311 miles), JCS said in a statement, adding South Korean and US intelligence were analysing the launch "in consideration of recent trends related to North Korea's missile development".

Early Monday, North Korean state media said it had conducted an "important final-stage test for the development of (a) reconnaissance satellite" at the Sohae Satellite Launch Ground, which is located in Tongchang-ri.

The North tested a "high-thrust solid-fuel motor" at Sohae on Thursday, with state media describing it as an important test "for the development of another new-type strategic weapon system".

"Given that the missiles launched today are medium-range ballistic missiles, it is assessed to be test-firings of a new ballistic missile equipped with the solid-fuel engine tested on December 15," said Cheong Seong-chang, a researcher at the Sejong Institute.

The South's military "strongly" condemned Sunday's launch, calling it a "serious provocation" and a "clear violation" of UN Security Council resolutions.

"Our military will maintain a firm readiness posture based on the ability to carry out an overwhelming response to any provocations by North Korea," it added.

- Kim's wish list -

Despite heavy international sanctions over its weapons programmes, Pyongyang has built up an arsenal of intercontinental ballistic missiles (ICBMs).

All its known ICBMs are liquid-fuelled, however, and North Korean leader Kim Jong Un has placed strategic priority on developing solid-fuel engines for more advanced missiles.

Liquid-fuel rockets are notoriously difficult to operate and take a long time to prepare for launch, making them slower and easier for the enemy to spot and destroy.

The more mobile solid-fuel missiles have a much shorter prep time, and are harder to detect before launch.

A wish list Kim revealed last year included solid-fuel ICBMs that could be launched from land or submarines.

The latest motor test was a step towards that goal, but it is not clear how far North Korea has come in the development of such a missile, analysts said.

- Key party meeting -

The isolated country's policy direction for next year will be laid out at a key party meeting later this month, and the official Korean Central News Agency earlier reported Kim saying that 2023 would be a "historic year".

In past years, Kim had delivered a speech every January 1, but he recently dropped the tradition in favour of making announcements at the year-end plenary meeting.

In his most recent address to the meeting, which was released to the public last New Year's Day, Kim focused on domestic affairs.

Experts say while Kim refrained from directly addressing the United States last year, he could change his tone this time around.

Kim said this year that he wants the North to have the world's most powerful nuclear force, and declared his country an "irreversible" nuclear state.

The United States and South Korea have warned for months that Pyongyang is preparing to conduct its seventh nuclear test.

North Korea is under multiple UN Security Council sanctions over its nuclear and missile activity since 2006.

North Korea conducts 'final-stage test' for spy satellite

North Korea carried out an "important final-stage test" for the development of a spy satellite, which it will complete by April next year, state media said on Monday.

The report comes a day after Seoul's military said it had detected launches by Pyongyang of two medium-range ballistic missiles, the North's latest in a year of unprecedented weapons tests.

Analysts say developing such a satellite would provide North Korea with cover for testing banned intercontinental ballistic missiles (ICBMs), as they share much of the same technology.

The launches were "an important final-stage test for the development of (a) reconnaissance satellite", a spokesperson with the North's National Aerospace Development Administration (NADA) said, according to the official Korean Central News Agency on Monday.

Conducted from Sohae Satellite Launching Ground in Tongchang-ri, Sunday's test confirmed "important technical indices" including camera operation in space, and data processing and transmission capabilities of communication devices .

State media also said the vehicle carrying the "test-piece" satellite -- which included cameras, image transmitters and receivers, control devices and batteries -- reached an altitude of 500 kilometres (311 miles) when it was fired at a high angle.

"The NADA said this is an important success which has gone through the final gateway process of the launch of (a) reconnaissance satellite," the spokesman said, adding that preparations will be completed by April.

Rodong Sinmun, the official newspaper of North Korea's ruling Workers' Party, carried two black-and-white photographs that appeared to show South Korea seen from space.

The development of a military reconnaissance satellite was one of Pyongyang's key defence projects outlined by North Korean leader Kim Jong Un last year.

Pyongyang carried out two launches earlier this year, claiming it was testing components for a reconnaissance satellite, which the United States and South Korea said likely involved components of its new Hwasong-17 intercontinental ballistic missile.

- Nuclear state -

On Thursday, the North tested a "high-thrust solid-fuel motor" at the Sohae launch site, which analysts say would allow quicker and more mobile launches of ballistic missiles.

All of Pyongyang's known ICBMs are liquid-fuelled, and solid-fuel ICBMs that can be launched from land or submarines are on Kim's wish list revealed last year.

Kim, who has doubled down on his banned weapons programmes since nuclear talks collapsed in 2019, said this year he wants the North to have the world's most powerful nuclear force, and declared his country an "irreversible" nuclear state.

The United States and South Korea have warned for months that Pyongyang is preparing to conduct its seventh nuclear test.

North Korea is under multiple UN Security Council sanctions over its nuclear and missile activity since 2006.